1. Field of the Invention
The present invention relates generally to a liquid crystal projection apparatus, and more specifically, to a liquid crystal projection apparatus which comprises a plurality of active matrix-type liquid crystal display panels or liquid crystal display panels of a simple matrix time division driving type, and projects and displays pictures of those display panels in such a manner that they are superimposed on a screen. The present invention also relates to a driving method of such a liquid crystal projection apparatus.
2. Description of the Background Art
FIG. 1 is a typical circuit diagram showing a structure of a general active matrix type liquid crystal display panel.
As shown in FIG. 1, in a liquid crystal display panel 10, a plurality of source lines S.sub.1 to S.sub.4 and a plurality of gate lines G.sub.1 to G.sub.4 are arranged intersecting with each other, and a thin film transistor 1 and a liquid crystal picture element 2 are provided at the respective intersections thereof. More specifically, one electrode of a liquid crystal picture element 2 is connected to a source line S.sub.1 through a thin film transistor 1, a gate of which is connected to a gate line G.sub.1. Similarly, the other picture elements are connected to the corresponding source lines through the thin film transistors, the gates of which are connected to the corresponding gate lines. The counter electrodes 3 of all the liquid crystal picture elements 2 receive common counter electrode signals COM. corresponding to a data signal. From a scanning signal generating circuit 8, gate signals G.sub.1 to G.sub.4 corresponding to vertical synchronizing signals are applied to the gate lines G.sub.1 to G.sub.4 The gate lines G.sub.1 and G.sub.3 correspond to the odd lines of TV signals and the gate lines G.sub.2 and G.sub.4 correspond to the even lines of the same. Meanwhile, a liquid crystal picture element 2 is equivalently shown as a capacitor.
FIG. 2 is a voltage waveform diagram showing an example of signal voltages applied to each portion of the liquid crystal display panel shown in FIG. 1.
S represents an analog sample and hold signal corresponding to a video signal voltage, V represents a vertical synchronizing signal, COM represents a counter electrode signal applied to a common counter electrode 3 of a liquid crystal picture element 2 and L represents a display voltage signal applied to a liquid crystal picture element 2 when the aforementioned signals have been applied thereto. Therefore, voltage signals of the same polarity are also applied to the other liquid crystal picture elements corresponding to the video signal voltages.
In an odd field, a scanning signal (a line sequential pulse signal based on a horizontal synchronizing signal) drives the gate lines G.sub.1 and G.sub.3 sequentially, and data signal voltages are applied to the liquid crystal picture elements through the source lines S.sub.1 to S.sub.4. At this moment, the gate lines G.sub.2 and G.sub.4 are in the off state. In an even field, the gate lines G.sub.2 and G.sub.4 are sequentially driven by a scanning signal as in the odd field, and data signal voltages are applied to the liquid crystal picture elements through the source lines S.sub.1 to S.sub.4. At this moment, the gate lines G.sub.1 and G.sub.3 are in the off state and the data signals in the odd fields are held. Then the polarities of the applied data signals are inverted for every frame and the liquid crystal picture elements are driven sequentially.
In FIG. 2, it is assumed that an analog sample and hold signal S applied to source lines fluctuates in the voltage range of +V.sub.1 to +V.sub.2. Counter electrode signals COM of +V.sub.4 and +V.sub.5 voltages are applied to the common counter electrodes of the liquid crystal picture elements alternatingly on a field-to-field basis. Therefore, the voltages of display voltage signals L applied to the liquid crystal picture elements are either +V=V.sub.1 -V.sub.5 or -V=V.sub.2 -V.sub.4 (V.sub.2 &lt;V.sub.4) for each field.
In connection with general TV pictures displayed by an interlace scanning method, when a conventional liquid crystal projection apparatus, in which general active matrix-type liquid crystal display panels are driven by field inversion, or frame inversion is employed, the driving frequency of the liquid crystal picture elements is equal to one half of the frame frequency or 15 Hz in case of TV signals of NTSC system. Therefore, a distribution in the spectrum of optical responses can be seen, which has a basic spectrum of 30 Hz or 15 Hz, which may cause flickers preventing practical use of the apparatus.